Clinical analyzers have for years provided rapid and accurate tests using slide-like test elements, as described, for example, in U.S. Pat. No. 4,298,571. Incubators have been used in such analyzers, along with preheat stations, to control the temperature of the test element while a patient sample drop spotted thereon undergoes a chemical reaction to produce a detectable change. That is, a test element is transferred from a spotting station to a preheat station, to an incubator, and then to a change-detecting station.
In most of the stations following the spotting station, a cover is used to contact the spotted test element to prevent significant evaporation of the sample. Such a cover is illustrated in FIG. 4 of the aforesaid '571 patent. Because some of the test elements are potentiometric types that have sample and/or reference drops protruding from the test element for a long time, as shown in said FIG. 4, the cover is grooved from side to side to allow clearance of such a drop. There is no need and no provision, however, in such covers, for the grooves to run from the exterior edge at which the test element enters, lengthwise along the direction of movement of the test element, since a piston is used to raise and lower the cover. Instead, the groove runs perpendicular to the direction of slide movement. Furthermore, the groove does not extend to any edge surface of the cover.
Such covers with such undercut grooves have been used for potentiometric test elements, because the drops on such elements have protruded above the rest of the test element. Such protrusion after spotting has not been a significant problem with most colorimetric elements, at least, not the type also useful with the analyzer of the '571 patent, since sample absorption is much more rapid in such colorimetric elements.
A problem has arisen, however, in the design of new analyzers that are to operate at higher throughput speeds to provide increased efficiency. Such new analyzers require a preheat station that receives colorimetric or rate test elements so soon after spotting, e.g., 500 millisec afterwards, that the drop still protrudes above the test element. Conventional covers in stations following the spotting station, e.g., the preheat station, have been unsatisfactory since they are designed to fit with an undersurface that is in flush contact with the test element. Such flush fit means the drop is wiped onto the undersurface as the test element is advanced into the particular station. This in turn produces unacceptable contamination of the cover and loss of sample volume. The first attempt at solving the problem was to provide a longitudinal groove extending the full length of the cover. This, however, was a failure due to the rapid evaporation that such a groove allowed to the drop.
Therefore, there has been a need prior to this invention to meet three competing goals: to provide a cover undersurface constructed in a way that does not wipe the drop of a colorimetric element, to keep such a drop from experiencing significant evaporation, and to minimize gaseous carryover.